The electric power steering system, such as mounted in the automotive vehicle, includes the electric motor coupled with a steering mechanism extending from a steering member to dirigible wheels and is designed to apply a power of the motor to the steering mechanism via a reduction mechanism thereby assisting the driver at operating the steering member. In such a steering system, rattle produced at meshing engagement portion between teeth portions included in the reduction mechanism may be transmitted to the driver as rattle noises (rattling noises) so as to cause discomfort to the driver. It is therefore desired to reduce the rattling noises.
Some conventional systems, such as disclosed in Japanese Unexamined Patent Publication No.2000-43739, are arranged as follows to meet this demand. In the reduction mechanism, a worm shaft coupled with an output shaft of the electric motor is urged toward a worm wheel mounted to the steering mechanism thereby accommodating backlash at the meshing engagement portion between the worm wheel and the worm shaft and suppressing the rattling noises.
Other conventional systems, such as disclosed in Japanese Unexamined Patent Publication No.2003-63424, use a lubricant as follows for reducing the rattling noises. That is, a permanent magnet is disposed in the vicinity of the meshing engagement portion, whereas the lubricant composed of a magnetic fluid magnetically attracted by the magnet is allowed to reside at the meshing engagement portion at all times such that the rattling noises may be reduced by way of the lubricant.
However, the aforementioned conventional systems do not pay adequate consideration to a vibration system including the reduction mechanism and the electric motor. Hence, the systems may sometimes encounter rattling noises caused by intrinsic vibrations in the vibration system or may suffer degraded steering feeling due to the intrinsic vibrations. Specifically, the electric motor as an inertial component is coupled with the steering mechanism via the meshing engagement portion between the teeth portions, as an elastic component, in the reduction mechanism, so that the aforesaid vibration system is constituted which has its reference end defined by the steering mechanism and its free end defined by the electric motor. When disturbance from the dirigible wheels is inversely inputted to the system, for example, the vibration system is prone to vibrations (resonance) particularly when the steering is in a dead zone where the electric motor is not driven. This may result in increased rattling noises or a significant degradation of the steering feeling.
In view of the aforementioned problem of the prior art, it is an object of the invention to provide an electric power steering system which is adapted to prevent the occurrence of the rattling noises attributable to the vibration system including the reduction mechanism and electric motor and to suppress the degradation of the steering feeling.